This invention relates to a pattern transfer technology for fabricating a semiconductor device and, more particularly, to a reticle used in a reduction projection aligner.
The reticle is a transparent glass plate, and a major surface of which is selectively covered with non-transparent metal film such as, for example, chromium. The reticle is broken down into a transparent portion and a non-transparent portion, and the boundary between the transparent portion and the non-transparent portion defines a pattern to be transferred to a photo-sensitive layer.
The pattern is formed in a central area of the major surface of the reticle, and the peripheral area around the pattern is covered with the non-transparent metal film. When a semiconductor device manufacturer transfers the pattern to a photo-resist layer spread over a semiconductor wafer, the reticle is mounted on a reticle stage in a reduction projection aligner, and is aligned with an optical path from a light source to the semiconductor wafer on a wafer stage. The reticle is illuminated with the light, and the image of the pattern is transferred from the reticle to the photo-resist layer so as to form a latent image in the photo-resist layer. Thus, the image is produced through the central area of the reticle.
If the reticle has pattern defects and/or pin-holes in the non-transparent metal film in the peripheral area, the defects are also transferred to the photo-resist layer, and the latent image contains images of defects. The latent image is developed, and the photo-resist layer is formed into a photo-resist mask on the semiconductor water. While the latent image is being developed, the images of the defects are influential in the pattern of the photo-resist mask. The photo-resist mask also contains the images of the defects. Using the deformed photo-resist mask, the images of the defects are further transferred to a semiconductor layer or an insulating layer under the photo-resist mask, and such a defect pattern is causative of a short-circuit or disconnection in an integrated circuit on the semiconductor chip. Thus, the defects on the reticle are transferred to the conductive/insulating pattern on the semiconductor wafer, and damages the integrated circuit on the semiconductor chip.
In order to prevent the conductive/insulating pattern of the integrated circuit from the images of the defects, the reticle is subjected to an inspection before the installation in the reduction projection aligner. If the inspection is carried out over the entire surface of the major surface, large amount of time and labor is consumed. The manufacture forms a slit 3 in the non-transparent metal film 2 in the peripheral area as shown in FIG. 1. Typical examples of the reticle formed with the slit 3 are disclosed in Japanese Patent Publication of Unexamined Application Nos. 62-219941 and 3-238455. The slit 3 loops around the pattern in the central area, and defines an area to be inspected. The manufacturer carries out the inspection inside the slit 3, and reduces the time and labor consumed for the inspection.
The slit 3 defines a pattern. If the slit pattern is transferred to the photo-resist layer, the slit pattern serves as a defect, and damages the integrated circuit. For this reason, it is necessary to design the slit 3 to have the width W2 narrower than the minimum width at the maximum resolution of the optical pattern transfer system on the photo-resist layer. However, the minimum width has been decreased. The minimum width is presently extremely narrow. This means that the width W2 is also extremely narrow. In this situation, the pattern recognition system hardly discriminates the slit 3 in the inspection.
It is therefore an important object of the present invention to provide a reticle, which has a pattern indicative of a boundary clearly recognized in the inspection.
To accomplish the object, the present invention proposes to form plural slits arranged at intervals narrower than the minimum width.
In accordance with one aspect of the present invention, there is provided a reticle used for a pattern transfer to a photo-sensitive layer comprising a transparent substrate having a major surface, a main pattern formed on the major surface, a non-transparent layer formed on the major surface outside of the main pattern and a discriminative pattern formed in the non-transparent layer for indicating a boundary of an area to be inspected for defects influential in a latent image to be formed in the photo-sensitive layer, having a width equal to or greater than a minimum width to be recognized by a pattern recognition means, and including plural portions arranged at intervals each less than a minimum width at a maximum resolution in a pattern transfer.